Direct positive films are used chiefly in reproduction technology because they enable the expert in the field to obtain a duplicate of the original rapidly without the necessity of producing intermediate photographs or intermediate negatives. Thus, for example, direct positive films are used for the production of transparent covers, clear intermediate copies, as well as duplicates of line and screen positives or negatives.
Photographic films have been produced which contain developer substances in the photographic film and which can be developed through simple treatment with a stable alkaline solution, the so-called "activator bath." The activation process can take place in table processors in very short times.
Since rapid processing is also desirable in the image reproduction industry, it would be desirable also to have available direct positive materials suitable for fast processing. It is generally known that the introduction of developer substances in photographic films presents considerable difficulty. For many applications, especially with transparent materials, density values of about 3 are necessary, so that highly active developers must be used.
In practice, the most frequently used high-activity superadditive developer combinations are hydroquinone/N-methyl-p-aminophenol sulfate and hydroquinone/1-phenyl-3-pyrazolidone. However, if these compounds are added to the photographic film, in general, photographic materials are obtained with an unsatisfactory stability on storage.
The difficulties are even more pronounced when developer substances are incorporated in direct positive emulsions, since a reciprocal influencing between silver fog and desensitizers occurs on the one hand and the developer on the other.
Thus, it is to be expected that the developer substance augments the fogging, in that development begins in the photographic film without activation, and as a result the later activation can no longer take place with sufficient differentiation, so that excessively high D.sub.min values are obtained. Beyond this, the material loses considerably in speed. Finally, developers and prefogged developer-containing direct positive materials can interact in such a way that a bleaching of the silver fog appears, so that in the activation only insufficient D.sub.max values are obtained, whereby in practice too high D.sub.min values appear at the same time. Moreover, the interaction between developer substances and desensitizers can lead to a decrease in speed by an order of magnitude compared to materials not containing developer. The interactions described are mostly time-dependent and, therefore, in practice lead to a negative effect on the storage stability.
From German Pat. No. 1,296,000 a process is already known for producing developer-containing direct positive materials, in which a colloidal layer containing a developer substance is applied to the dried direct positive emulsion layer, and whereby this layer must be dried within at most 50 seconds. Aside from the fact that this process requires special casting conditions, practice has shown that in the processing, a large part of the developer contained in the topcoat film does not diffuse to the silver halide grains but diffuses directly into the activator bath, so that a sufficient density value is not achieved.
Therefore, a direct positive material containing a developer substance is needed which in the activation gives direct positive images with good sensitometric values, that is, high D.sub.max values and low D.sub.min values. At the same time, the material must have good storage stability.